The invention relates to shears for cutting billets and bars of large cross-section, and more particularly to shears having two shearing blades which are able to move relative to each other and are located in blade holders, and with a crank drive for one blade holder, which is connected by way of a press rod to an eccentric of the crankshaft.
In known shears designed for cutting hot blocks and slabs (German OS No. 1 550 875), a continuously rotating crankshaft is provided. The crankshaft forms the driving member for one blade holder, so that for each revolution of the crankshaft, one blade holder carries out an up-and-down movement according to the offset of the eccentric. The second blade holder is stationary and is not moved, at least during the working or cutting stroke. The known shears are operated so that in addition to the drive for the crankshaft, a further driving device is incorporated, which comes into use during a return stroke of the eccentric. This second driving device adjusts or regulates the spacing of the two blades with respect to each other in the shear frame, due to which the shear blade is gradually moved in the direction of the other shear blade until the cut is achieved. The purpose and aim of these shears is to provide an eccentric with an offset only a fraction of the total stroke necessary for cutting and then to actuate this eccentric several times in the course of a parting cut, while the shear blade is moved gradually. Thus, a reduction in driving power is made possible due to the low moment of inertia (GD.sup.2). Accordingly, for reasons of cost, a motor with lower torque may be used. This, however, is detrimental to production, since a plurality of working strokes is required for the final separation of the material.
To effect the continuous or gradual cutting movement of the movable blade holder, in addition to an extra drive required for a screw-down movement, a special coupling is needed between the motor and drivetransmission device. The coupling should allow rotation in only one direction, in order to prevent the drive for the screw-down movement to allow the shears to reverse direction due to the restoring force. Furthermore, the shears described are not suitable for cutting cold billets and bars. In cold billets and bars, a plurality of working strokes produce compression and consolidation of material at the cutting point during each stroke. This necessitates an enormously increased shearing force, which would ultimately lead to a need for increased performance and more expensive drives.
It is known to use so-called starting shears for cutting billets and bars. This type of shears has a crank stroke of the drive shaft which is sufficient to cut through the material in one working stroke. However, one drawback of these shears is that the starting shears must be started up on each occasion from an inoperative position and apply the force necessary for cutting to the shear blades within a short time, for example, six working strokes per minute every ten seconds. This means that there is a high power requirement which necessitates expensive driving devices in order to be able to achieve, in a short time, the high torque which is necessary at the low speeds of the eccentric shaft.